1. Industrial field
This invention relates to a magnetic bearing device.
2. Prior art
FIG. 5 shows an example of known magnetic bearing devices conventionally used, and in which reference numeral 1 indicates a rotor to which an armature disk 2 is rotatably attached. A pair of bearing elements 3, 3 are disposed at positions on opposite sides in the axial direction putting the armature disk 2 therebetween. Each of the bearing elements 3, 3 comprises a pair of pole pieces 5, 6 attached to two pole faces of a premanent magnet 4. Each of the pole pieces 5, 6 comprises a part extending in parallel to the armature disk 2 and a top end part extending from the mentioned part to the position adjacent the armature disk 2. A control coil 7 is disposed at a position nearer to the top end part than the permanent magnet 4 between the mentioned pair of pole pieces 5, 6. A sensor coil 8 is disposed at a position nearer to the top end part than the control coil 7. The sensor coil 8 is wound in the same direction as the mentioned control coil 7, such that a bridge is formed between a pair of upper and lower sensor coils 8, 8 so as to differentially output variations of distance between the coil 8 and the armature disk 2.
The known magnetic bearing device of the above construction is controlled with a control current of the control coil 7 in the following manner. For carrying out a simple control as shown in FIG. 6, supposing that control current is I, displacement of the armature disk 2 is x and weight of the rotor 1 is mg, the force acting on the armature disk 2 is expressed as follows: EQU F=a.multidot.I.sup.2 /x (1)
(where "a" is a proportional constant)
Accordingly, the condition to be F=mg is expressed as follows: ##EQU1## This means that the current I required in the control coil 7 varies according to the position x where magnetic attraction of the control coil 7 and weight of the rotor 1 are balanced.
Accordingly, the control current I is controlled so as to be I=k.multidot.x in the prior art. This is rewritten as x=I/k, and when substituting this expression for x in the above expression (2), a following expression (3) is obtained: EQU I=mg/a.multidot.k (3)
It is obviously understood from tha above expression (3) that the rotor 1 can be magnetically supported by a smaller current when establishing the gain k to be larger.
Thus, it seems desirable that the gain k for establishing the control current I be as large as possible.
When establishing the gain k to be as large as possible, however, there arises the serious disadvantage of narrowing the stable region of the rotor 1 with increased gain k. More specifically, establishing a maximum allowable current of the power supply for control current as I, magnetic attraction of the control coil 7 is lowered in the region of x&gt;I.sub.o /k as shown in FIGS. 7(a) and (b), which results in unstable magnetic support.
This invention was made to solve the above-discussed problem and has an object of providing a magnetic bearing device in which small control current is satisfiable yet preventing from the disadvantage of enlargement of unstable region.
Other objects of the invention will become apparent in the course of the description of embodiment later-described.